1. Technical Field
The present invention relates generally to earth boring bits, and more particularly to a rotary drag bit impregnated with diamond.
2. Description of Related Art
Impregnated drill bits typically employ a cutting face composed of superabrasive cutting particles, such as natural or synthetic diamond grit, dispersed within a matrix of wear-resistant material. As such a drill bit is operated to drill a formation, the matrix and embedded diamond particles wear, worn cutting particles are lost and new cutting particles are exposed. These diamond particles may either be natural or synthetic and may be cast integral with the body of the bit, as in low-pressure infiltration, or may be preformed separately, as in hot isostatic pressure infiltration, and attached to the bit by brazing or furnaced to the bit body during the manufacturing by an infiltration process.
Reference is now made to FIG. 1 which shows a prior art impregnated bit 10. This bit is made with an aggregate of diamond and matrix powder which is infiltrated. The diamond particles are cast within a supporting material to form an abrasive layer. During operation of the drill bit, diamonds within the abrasive layer are gradually exposed as the supporting material is worn away.
It is important that the bit 10 support the movement of fluid in order to remove rock cuttings and cool the cutting surface of the bit. To this end, the cutting face of the impregnated bit includes an arrangement of radially extending fluid channels 12. The fluid channels 12 divide the abrasive surface of the bit 10 into a plurality of distinct raised ribs 14. The abrasive cutting surface of the bit 10 is defined by the top surface of the ribs 14. All or a portion of a rib 14′ may be removed at or near the gage of the bit 10 to define a gap 16. The gap 16 provides a wider fluid course than is provided by any one of the fluid channels 12.
A drilling fluid is pumped down the drill string and through a central plenum of the bit 10 to pass out through openings to the cutting surface of the bit. Rock cuttings generated by the abrasive contact of the top surface of the ribs 14 with the formation being cut mix with the drilling fluid to form a slurry. This slurry passes through the fluid channels 12 and gaps 16 to clean and cool the ribs 14 before passing back up to the surface in an annulus formed between the drill string and the drill hole. The presence of a combination of fluid channels 12 and gaps 16 assists in the efficient evacuation of rock cuttings from the bottom of the drill hole.
An impregnated drill bits is typically made from a solid body of matrix material formed by any one of a number of powder metallurgy processes known in the art. During the powder metallurgy process, super abrasive particles and a matrix powder filling a mold cavity are infiltrated with a molten binder material. Upon cooling, the bit body includes the binder material, matrix material, and the super abrasive particles suspended both near and on the surface of the drill bit. The super abrasive particles typically include small particles of natural or synthetic diamond. Synthetic diamond used in diamond impregnated drill bits is typically in the form of single crystals. However, thermally stable polycrystalline diamond (TSP) particles may also be used.
To connect to the drill string, the impregnated bit must include a drill blank/shank. The shank of the bit is supported within the mold cavity along with any necessary former (such as would be used to define holes for fluid passage or the inclusion of diamond impregnated inserts or other cutting structures). The remainder of the cavity is then filled with a charge of tungsten carbide powder. A binder referred to an infiltrant (such as a nickel brass copper based alloy) is placed on top of the charge of tungsten carbide powder. The mold is then heated sufficiently to melt the infiltrant and held at an elevated temperature for a sufficient period to allow the infiltrant to flow into and bind the powder matrix or matrix and segments. A monolithic bit body is accordingly formed.
The center 18 of the face of the impregnated drill bit 10 (at or about the axis of bit rotation) is a critical area of the bit recognized by those skilled in the art to have a durability concern. There is a need in the art for an improved impregnated bit design which addresses the durability concern at the bit center.